The weight of a vehicle influences several functions of the vehicle. Safety and assistance systems like ABS or ESP may be optimised by accurate knowledge of the vehicle weight. For example, anti-skid systems control brake actuation so that the wheels of the vehicle do not lock by releasing the brake when locking is detected. Thereby control strategy is improved by proactively monitor several parameters to optimise the brake actuation. Beside several other parameters like wheel to ground contact condition, the tendency for the wheels to lock depends on the vehicle weight which has to be decelerated so that an accurate knowledge of the current vehicle weight helps to improve the anti-skid strategy. This improves safety and reduces wear on the brakes and wheel tyres.
The vehicle weight is especially important for vehicles which may vary significantly in weight due to changing payload such as trucks and agricultural vehicles. In such vehicles weight may vary from 15 tons when empty to 40 tons when full loaded so that the weight may vary by about 300%. As described above, this is an important parameter to improve the control quality of anti-skid systems.
The prior art shows different procedures to determine vehicle weight which are discussed briefly below.
U.S. Pat. No. 6,526,334 describes a system in which the vehicle weight is determined by measuring wheel load on the respective wheels. The detailed procedure is not described but it is well known to use, e.g. tyre pressure or tyre deformation as an indication of vehicle weight. The procedure is costly as sensors must be installed solely for this purpose. In addition, this procedure must be seen as insufficient as a combination of a pulling vehicle and a trailer can only be controlled using this procedure if the trailer is also equipped with such costly sensors. This is not cost-efficient, as many different trailers are used especially in combination with tractors.
U.S. Pat. No. 5,938,295 describes a system in which the vehicle weight is determined by measuring wheel load by pressure sensors in the suspension system. Similarly to U.S. Pat. No. 6,526,334 the procedure requires sensors also in the trailer. In addition, not every tractor is equipped with suspension systems on the front, or even more rarely on the rear axle, so again, additional sensors must be installed increasing vehicle costs.
Trucks mainly determine vehicle weight by including torque of the combustion engine in combination with force impact during shifting.
U.S. Pat. No. 4,548,079 (EP 111 636) describes a procedure in which the torque of the combustion engine is combined with an acceleration sensor whereby rolling resistance and air resistance is included. The engine torque thereby delivers a value to calculate the pull force to drive and accelerate the vehicle. In addition, performance requirements of secondary drives, e.g. HVAC compressor or air compressor are included by using characteristic maps.
This procedure mitigates some of the problems mentioned above, especially the fact that trailers can be included into the calculation without requiring additional sensors. But on the other hand this procedure is only adequate for vehicles in which the engine torque is mainly transferred in driving performance and the influence of secondary drives, eg. HVAC compressor or generators are minor. This is especially correct for trucks in which secondary drives require very low performance or a performance which is known or easy to calculate in every operating condition.
Especially in agricultural vehicles, e.g. tractors, the influence of secondary drives is much higher as driving over ground is just a secondary purpose in some operations. Especially hydraulic supply systems need high performance which cannot be determined in every operating situation especially as trailers/implements are supplied.
So, considering engine torque as a major parameter to determine vehicle weight is not suitable for vehicles like agricultural tractors.